1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to an image forming apparatus of the type forming a multicolor visible image or toner image on an image carrier and then transferring it to a transfer medium at a time.
2. Discussion of the Background
An image forming apparatus of the type forming a multicolor toner image on an image carrier with developers of different colors is conventional. It has been customary with this type of apparatus to use, as a developer for developing a latent image of second color, a non-magnetic toner or one component type developer which colors easily and promotes a small size and low cost configuration. Such a toner is deposited in a layer on a developer carrier which faces, but does not contact, an image carrier. With this arrangement, it is possible to develop a latent image of second color without disturbing a toner image of first color existing on the image carrier.
Specifically, the above-described type of apparatus may be constructed as follows. A plurality of developing means are arranged around an image carrier. Among them, first developing means assigned to a first color stores a two component type developer containing a chromatic toner. Second developing means assigned to a second color is located downstream of the first developing means and loaded with a black one component type developer, i.e., black toner. The black toner is opposite in polarity to the toner of the first developing means. The toner of the second developing means forms a 30 .mu.m to 500 .mu.m thick layer on a developer carrier and is held out of contact with the image carrier. In the event of development, the developer carrier of the second developing means is applied with an AC bias voltage of the kind generating an AC electric field which causes the toner to develop a latent image. On the other hand, when the second developing means is out of operation, a bias voltage of the kind generating an electric field which causes the chromatic toner of the first developing means to develop a latent image is applied to the developer carrier of the second developing means. This kind of arrangement taught in, for example, Japanese Patent Laid-Open Publication (Kokai) No. 63-60471.
However, the problem with the above-described conventional apparatus is that the AC bias voltage for the development in the second color causes the toner of second color to move back and forth between the surface of the image carrier and that of the developer carrier while hitting against the former surface, disturbing the toner image of first color existing on the image carrier. Moreover, the toner of first color also moves back and forth together with the toner of second color while hitting against the surface of the developer carrier. As a result, the toner of first color is introduced into the second developing means, making the toner of second color turbid.
To prevent toners of different colors from being mixed together, a DC bias voltage may be applied in the event of development in the second color so as to cause the non-magnetic toner to fly toward the image carrier, as proposed in the past. Specifically, this kind of approach uses an image carrier having a photoconductive layer made of selenium or arsenic selenide. The photoconductive layer is 35 .mu.m to 90 .mu.m thick and has an electrostatic capacity of 20 pF/cm.sup.2 to 170 pF/cm.sup.2. A plurality of developing means are arranged around the image carrier. Charging, exposing and developing steps are repeated a plurality of times to form a multicolor image on a single image carrier. In second developing means assigned to a second color, a developer carrier is spaced less than 250 .mu.m from the image carrier and has applied thereto a DC bias voltage to effect non-contact development. At this instant, the other developer carriers not contributing to development are not driven to prevent the toners of different colors from being mixed. This type of apparatus is disclosed in Japanese Kokai No. 63-63061 by way of example. Kokai No. 63-63061 includes an embodiment using an image carrier having a 15 .mu.m to 50 .mu.m thick organic photoconductor layer, a scorotron charger, a reversal development procedure, a potential contrast of higher than 400 V, and a 5 .mu.m to 30 .mu.m thick toner layer deposited on the image carrier.
An image forming apparatus of the type forming a multicolor image with a plurality of developing means adjoining, but not contacting, a recording medium is also known in the art. In first or upstream development means, a developer carrier is applied with a DC-biased AC voltage and rotated in the same direction as and at a higher peripheral speed than the recording medium for effecting development in black. Downstream or second developing means and successive developing means are each applied only with a DC voltage and operated at equal speed to perform color development. For this type of apparatus, a reference may be made to Japanese Kokai No. 63-85578 by way of example. However, when the toner is caused to fly by an electric field generated by a DC bias voltage, the cohered toner is locally omitted in low contrast portions, resulting in a critically granular image. Moreover, in the line portions of a latent image, the edge electric field of the latent image turns round to the image carrier, preventing thin lines from being reproduced.